1. Field of the Invention
The present invention relates to an elastic coupling provided in a steering shaft of a steering device for a vehicle.
2. Description of the Related Art
As is known, in a structure of a steering device of a vehicle, a steering shaft connects a steering gear box provided on a cross member of a body of the vehicle with a steering wheel provided adjacent to a driver's seat, and rotation of the steering wheel is transmitted to the steering gear box via the steering shaft to steer right and left wheels by means of the steering gear box. The steering shaft is provided with an elastic coupling in order to prevent an input from a road, a shake (vibration) of an engine or the like from being transmitted to the steering wheel side (see JP 9-196077A, for example).
In a steering device disclosed in JP 9-196077A, a steering coupling as an elastic coupling is incorporated in an intermediate shaft connecting a steering gear box with a main shaft. The steering coupling comprises cylindrical metal fittings connected on the steering gear box side, shaft metal fittings connected on the main shaft side and a rubber elastic body provided between an inner circumference of the cylindrical metal fittings and an outer circumference of the shaft metal fittings. The inner circumference of the cylindrical metal fittings and the outer circumference of the shaft metal fittings are respectively formed into a roughly cross shape in section. The rubber elastic body has a corresponding shape in section and is provided between the both metal fittings. This allows rotation from the steering wheel to be transmitted to the steering gear box side through the rubber elastic body and a shimmy from the steering gear box side (a shake around a rotation shaft) to be absorbed by elastically changing a shape of the rubber elastic body.
As is known, a steering gear box is provided on a cross member of a body of a vehicle, and thereby, there is a phenomenon that the steering gear box is displaced up and down and right an left due to bending of a cross member per se or bending of a mounting portion of the steering gear box when the steering gear box receives an input from a road through a member such as a wheel to be steered, a tie rod or the like.
Such displacement is transmitted to a steering coupling as a shake in an axis direction. In the steering coupling disclosed in JP 9-196077A, however, there is a problem that the shake in an axis direction is transmitted since a shimmy, which is a shake around a rotation shaft, is only assumed. In more detail, a rubber elastic body of a steering coupling is vulcanized and adhered to an inner surface of cylindrical metal fittings while pressured and inserted in an outer circumferential surface of shaft metal fittings, so that it cannot move relatively to any of the cylindrical metal fittings and the shaft metal fittings. Therefore, an operation of absorbing a shake in an axis direction has only an effect that the rubber elastic body is slightly elastically changed in shape in an axis direction, so that most of the shake is transmitted as it is. As a result, there is a problem caused that a driver handling a steering wheel feels uncomfortable.